Electron optics for multi-beam electron beam lithography tool

ABSTRACT

A charge particle optical column capable of being used in a high throughput, mutli-column, multi-beam electron beam lithography system is disclosed herein. The column has the following properties: purely electrostatic components; small column footprint (20 mm square); multiple, individually focused charge particle beams; telecentric scanning of all beams simultaneously on a wafer for increased depth of field; and conjugate blanking of the charged particle beams for reduced beam blur. An electron gun is disclosed that uses microfabricated field emission sources and a microfabricated aperture-deflector assembly. The aperture-deflector assembly acts as a perfect lens in focusing, steering and blanking a multipicity of electron beams through the back focal plane of an immersion lens located at the bottom of the column. Beam blanking can be performed using a gating signal to decrease beam blur during writing on the wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/722,079 filed Nov. 23, 2000, which claims the benefit ofU.S. Provisional Application No. 60/167,442 filed Nov. 23, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to the field of lithography, and inparticular to lenses and other column components, suitable for use incharged particle beam direct-write lithography.

[0004] 2. Description of the Related Art

[0005] At present, there is no practical solution for next generationlithography (NGL) at the ITRS 70 nm resolution node. The leading NGLcontenders—Extreme Ultraviolet Lithography, Electron ProjectionLithography, X-Ray Lithography and Ion Projection Lithography—all usemasks. NGL masks are difficult to fabricate and expensive; and manylithography masks are required for standard IC chips, and in particularfor microprocessor chips. The latest Pentium III microprocessor requiresapproximately 30 masks. These mask costs must be amortized into the costof fabricating the IC chips. Electron beam direct-write (EBDW) systemsoffer two particular advantages over other NGL technologies: (1) theyare maskless, thus eliminating mask amortization costs and expeditingchip development cycles; (2) they have the capabilities of meeting allfuture ITRS nodes in terms of resolution (out to critical dimensions of25 nm). The primary disadvantage of the traditional single column andprobe-forming or shaped-beam systems is wafer throughput limitation dueto space-charge effects. Space-charge effects are electron-electroninteractions that occur when there are regions within the column withhigh electron beam current density. These effects tend to blur the beamand increase the spot size on the wafer. Since most electron opticalcolumn designs have a crossover, in which all of the electrons passthrough a small area, the current density becomes quite large. In orderto achieve writing resolution of less than 50 nm, electron beam currentsneed to be limited to roughly 1 μA through a crossover. For a 300 mmdiameter wafer and a 10 μC/cm² resist sensitivity, a simple calculationshows that an electron beam current of roughly 80 μA is required toexpose the entire wafer in a time of 90 seconds. Including a writingoverhead of 30 seconds, this results in a wafer writing throughput of 30wafers/hr, which barely meets the chip manufacturers' throughputrequirements. A more sensitive resist can be used, but then statisticaldose issues become a concern. As can be seen, this amount of electronbeam current (80 μA) is much too high to be used in a single columnapproach with high resolution. In order to keep the column current toless than 1 μA/column, a minimum of 80 beams that do not interact witheach other are needed. Thus, a multi-beam approach is required.

[0006] The straightforward technique to reduce space-charge effects isto spread the current over the wafer by using multiple beams that writesimultaneously. Some have proposed multi-beam systems using multiplecolumns and only a single beam per column, such as Chang, et al. [T. H.P. Chang, D. P. Kern, and L. P. Murray, J. Vac. Sci. Tech. B 10(6), pp.2743 (1992)] and Groves and Kendall [T. R. Groves and R. A. Kendall, J.Vac. Sci. Technol. B 16(6), 3168, (1998)]. Others, such as Yasuda [H.Yasuda et al., J. Vac. Sci. Technol. B 14(6), 3813 (1996)] and Schneider[J. E. Schneider, P. Sen, D. S. Pickard, G. I. Winograd, M. A. McCord,R. F. W. Pease, W. E. Spicer, A. W. Baum, K. A. Costello, and G. A.Davis., J. Vac. Sci. Technol. B 16(6), 3192 (1998)] propose usingmultiple beams within a single column. However, these approaches runinto another major problem for EBDW systems: data rate. Because the datarate is applied serially to each writing beam, extremely high data ratesare required for typical EBDW systems. Assuming that each beam isblanked individually, the data transfer rate of the pattern onto thewafer can be calculated, and from this calculation, we determine anappropriate number of beams required. For a 300 mm wafer with 25 nm×25nm pixels, there are a total of 1.1×10¹⁴ pixels on the entire wafer. Inorder to write the wafer in 90 seconds, an overall data rate of1.26×10¹² pixels/s is required. Blanking rates on the order of 100-300MHz are presently achievable. Therefore, the minimum number of beamsthat satisfy the blanking rate requirement is between 4,000 and 12,000.With a practical blanking rate of 250 MHz per beam, roughly 6000individually controllable beams are required. An approach having 6000columns per wafer, or 6000 beams per column is not realistic, both interms of fabrication and electrical interconnects. A multiple columnapproach, with each column having multiple beams, would solve thisproblem.

[0007] To achieve a compact design with multiple beams per column in amultiple column system is quite challenging. To focus an electron beamto high resolution, without aberrations that cause degradation in thebeam shape and size, requires a uniform electrostatic or magnetic field.This level of field uniformity is typically achieved only if thediameter of the electrostatic lens bore is roughly 10 to 100 timeslarger than the diameter of the electron beam passing through the middleof the lens. Because electron optic imaging systems typically also havea large demagnification factor, this results in a writing field of viewon the wafer that is much smaller than the lens diameter. For example,if the writing area required on the wafer is roughly 250 μm, and thedemagnification of the imaging system is roughly {fraction (1/50)}×,then the lens bore diameter must be in the range of 125 mm to 1.25 m indiameter in order to minimize aberrations. Since the wafer diameteritself is only 300 mm, this standard electrostatic lens cannot be usedin a multi-column approach. A more compact lens design that canindividually focus multiple beams within a single compact column designwould overcome this problem.

SUMMARY OF THE INVENTION

[0008] This invention includes lenses and other column components,suitable for use in multiple charged particle beam systems, andparticularly in multiple column, multiple charged particle beam systems.According to aspects of this invention, an integrated optical elementfor independent alignment of multiple charged particle beams comprises:a substrate for providing structural support with a multiplicity ofapertures and a multiplicity of independently addressable alignmentdeflectors situated over insulating material of the substrate, such thateach of the deflectors is positioned over a corresponding substrateaperture. Further, a multiplicity of object apertures can be situatedover the deflectors, such that each object aperture is positioned overand electrically isolated from a corresponding deflector. Furthermore, amultiplicity of independently addressable blankers can be situated overthe deflectors, such that each blanker is positioned over andelectrically isolated from a corresponding deflector. Further, amultiplicity of spray apertures can be situated between the substrateand the deflectors, such that each spray aperture is positioned belowand electrically isolated from a corresponding deflector. Themultiplicity of deflectors can be arranged in a regular array, such as aline. In preferred embodiments the integrated optical element comprisesspray apertures, deflectors, object apertures and blankers, as describedabove, with the blankers situatedover the object apertures such thateach blanker is positioned over and electrically isolated from acorresponding object aperture.

[0009] According to further aspects of the invention, an optical columnfor multiple charged particle probe generation comprises: a chargedparticle source for generating a multiplicity of charged particle beams;an integrated optical element for independent alignment of each chargedparticle beam; an accelerating column; a deflector; a blanking aperture;and an immersion lens. Further, the optical column can include a rotatorbetween the integrated optical element and the accelerating column. Thecharged particle source can be a multiplicity of field emissioncathodes; the source can also be an ion source. The charged particlesource and the integrated optical element can be bonded together. Theoptical column can also include means for gated blanking, electricallyconnected to the blankers in the inegrated optical element.

BRIEF DESCRIPTION OF THE FIGURES

[0010]FIG. 1 shows a top plan view schematic of a multi-column layoutover a 300 mm wafer within a lithography writing unit.

[0011]FIG. 2 shows a schematic cross-section of an individual columnwithin a multi-column lithography writing unit.

[0012]FIG. 3 shows a schematic cross-section of 4 side-by-side columnswithin a multi-column lithography writing unit.

[0013]FIG. 4a shows a schematic representation of the column footprinton the wafer, each column having 32 beams.

[0014]FIG. 4b shows a magnified view of FIG. 4a, indicating theelectronic scanning of the 32 beams of a single column over the columnfootprint, and the scanning of the stage underneath the column.

[0015]FIG. 4c shows a schematic of a single stripe and its decompositioninto substripes.

[0016]FIG. 4d shows a magnified view of FIG. 4c, indicating a singlesubstripe and its decomposition into writing pixels.

[0017]FIG. 4e shows a magnified view of FIG. 4d, indicating the pixelexposure sequence across a single substripe.

[0018]FIG. 5 shows a top plan view schematic of a 4×4 column array,indicating the tip array orientation and the motion of the stage.

[0019]FIG. 6 shows a cross-sectional schematic of an electron gun,showing the current sense area and tip current regulation circuit.

[0020]FIG. 7 shows a plan view schematic of a non-equisector dodecapoledeflector.

[0021]FIG. 8 shows a schematic diagram of how a beamlet is deflected offthe blanking aperture using the blanking electrodes.

[0022]FIG. 9 shows plots of the blanking signal, gating signal, blankingfield, and beam current on the wafer versus time, as required forblanking using the gated blanker approach.

[0023]FIGS. 10a-10 o schematically illustrate the fabrication sequenceof an aperture-deflector assembly. All figures are schematiccross-sectional side-views of a single aperture-deflector structure.

[0024]FIG. 11 shows a schematic cross-section of 3 out of 32aperture-deflector assemblies on the aperture-deflector die.

[0025]FIG. 12 shows a plan view schematic of the source die andaperture-deflector die, indicating the bond pads and silicon oxynitrideinsulating pedestals.

[0026]FIG. 13 shows a cross-sectional view of the source andaperture-deflector assemblies bonded together.

[0027]FIG. 14 shows an isometric view of the rotators acting on 5 out of32 beamlets, and a schematic plan view of the rotator electrodes.

[0028]FIG. 15 shows a schematic diagram of how a beamlet is doubledeflected onto the wafer by the mainfield deflectors.

[0029]FIG. 16 shows a schematic diagram of how a beamlet is scanned onthe wafer by the subfield deflector.

[0030]FIG. 17a shows a schematic representation of how pixels arewritten on the wafer, without stage motion correction using the subfielddeflectors.

[0031]FIG. 17b shows a schematic representation of how pixels arewritten on the wafer, with stage motion correction using the subfielddeflectors.

[0032]FIG. 18a shows a schematic cross-section of an on-axis beamletbeing focused by the immersion lens onto the wafer.

[0033]FIG. 18b shows a schematic cross-section of an off-axis beamletbeing focused by the immersion lens onto the wafer.

[0034]FIG. 19 shows a bottom plan view schematic of a 4×4 column array,indicating how the BSE detectors are mounted in the bottom of the lensplate.

[0035]FIG. 20 shows a schematic cross-section of the column electronoptical components and a block diagram of the column controlelectronics.

DETAILED DESCRIPTION

[0036] Consider a multiple column, multiple beam electron beamlithography system. This system has the advantages of masklesslithography, while overcoming the two fundamental limits of typicalelectron beam direct-write (EBDW) systems: space charge effects and highdata rates. Space charge effects are overcome by introducing multiplecolumns that write simultaneously and spread the electron beam currentover the entire wafer. By eliminating space charge effects, smallwriting spot sizes can be achieved. The data rate problem is overcome byintroducing multiple beams per column to allow for a realisticallyachievable blanking bandwidth for each beam. Hence, EBDW in amulticolumn, multibeam system can be attractive in terms of highthroughput, high resolution and lowered cost of ownership inmanufacturing.

[0037] The specifications of each electron optical column must first bedetermined by examining the throughput and data rate requirements. Themain factors that determine the minimum number of beams in amulti-electron beam direct-write lithography system with high throughputare the blanking speed and data rate. From the considerations ofblanking rate, the minimum number of beams required is 6000. For a 300mm wafer and 25×25 nm pixels, this corresponds to a wafer throughput of30 wafers/hour (including overhead), and a blanking rate of 250 MHz.Next, the number of columns must be determined.

[0038] There are two main factors that determine the number of columnsrequired: space-charge effects and column footprint. Space-chargeeffects can become more significant when the column design includes acrossover, as is the case in most single-column, multi-beam approaches.The crossover region, which is a region of high current density, isknown to introduce Boersch effect (energy broadening). An optimizationof these various parameters leads to a column footprint (i.e., thecolumn dimensions projected into the plane of the wafer) of roughly 20mm×20 mm, requiring a total number of 201 columns to cover a wafer. Theminimum area of the column footprint is presently limited by the numberof connections required for each column. Each column has 32 beams, for atotal number of 6432 beams. Therefore, each beam must have an averagecurrent of roughly 12 nA (≅80 μA/6432, where 80 μA is the total currentrequired for a throughput of 30 wafers/hr with a resist sensitivity of10 μC/cm²).

[0039]FIG. 1 shows a schematic of the layout of 201 columns positionedover a 300 mm wafer within the lithography writing head, including thecolumn positions 100, wafer 102, row numbers 104, column numbers 106,and wafer stage axes 108. The lithography writing head is defined as thecollection of columns and their associated electronics that are requiredto write on a 300 mm wafer. As can be seen, the number of columns isderived from a 15×15 array of square columns, each of which has afootprint of 20×20 mm. The 6 columns in each corner are not requiredbecause of the round shape of the wafer; hence (15×15)−(6×4)=201columns. Examples of column notation in terms of rows and columns aregiven in FIG. 1. The stage axes and how they interact with the writingstrategy will be discussed in detail in the Writing Strategy sectionbelow. This is one embodiment of the lithography writing head columnlayout positioned over the wafer 102. Depending on the ease ofmanufacturability for different column sizes and interconnects, thecolumn footprint area can be adjusted. However, as discussed in theBACKGROUND OF THE INVENTION, a minimum number of roughly 80 columns isrequired in order to maintain a wafer throughput of 30 wafers/hour withless than 1 μA per column.

[0040] A schematic of the first embodiment of the invention, which is asingle electron optical column to be used in a multicolumn, multibeamlithography system writing head, is shown in FIG. 2. The schematic showsthe field emission tips 202, source substrate 204, gate electrodes 206,focus electrodes 208, focus shield electrode 210, spray aperture 212,alignment deflectors 214, object aperture plate 216, blanking electrodes218, individual beamlets 220, rotator 222, accelerating column plates224, shield electrode 226, mainfield deflectors 228, subfield deflectors230, blanking aperture or lens plate 232, blanking aperture 234,backscattered electron (BSE) detectors 236, immersion lens 238,backscattered electrons 240, wafer 242, positions 244 of the beamlets onthe wafer, and the electron gun 250. Only 3 out of the 32 electronsources in the preferred embodiment of the invention are shown in FIG.2. The functionality of each of these electron optical components willbe described below.

[0041]FIG. 3 shows the placement of 4 adjacent electron columns out ofthe 201 columns in the lithography writing head, indicating the electronguns 250, mainfield 228 and subfield 230 deflectors, BSE detectors 236,shield electrode 226, lens plate 232 and wafer 242. As can be seen fromFIG. 3, in the preferred embodiment of the invention, the electron guns250, mainfield 228 & subfield 230 deflectors, and BSE detectors 236 areindividually controllable within each column. However, the shieldelectrode 226 and lens plate 232 are common to all of the columns. Thisreduces the number of interconnects to the writing head.

[0042] Referring to FIG. 2, each electron optical column can be brokeninto 3 main sections: (1) the electron gun 250, consisting of a fieldemission source 202, focusing optics, alignment deflector optics andblankers 218; (2) the accelerating region and scanning deflectors,consisting of the rotator 222, accelerating plates 224, shield electrode226, mainfield deflectors 228 and subfield deflectors 230; and (3) theimmersion lens 238 and BSE detectors 236. A simplified view of thecolumn operation is as follows. The electron gun 250 creates 32individually controllable, focused electron beams, precisely steers eachbeamlet 220 individually down the column through the blanking aperture234, and individually blanks each beamlet 220 by slightly deflecting itoff the blanking aperture 234 and onto the lens plate 232. Theaccelerating region and deflectors increase the energy of the electronbeams and scan all 32 beamlets simultaneously on the wafer 242 to writeout the pattern input into the blanking electrodes 218. This region isalso used to correct for mechanical and stage error, as well asperforming some fine focusing adjustment for the 32 beamlets. Theimmersion lens 238 and BSE detectors 236 provide the primary focusingfor all 32 beamlets onto the wafer 242 and detect backscatteredelectrons 240 that are emitted from the exposed surface. The BSEdetectors 236 are used for alignment mark detection and alignment of thebeamlets 220 over the wafer 242.

[0043] Referring to FIG. 2, the preferred embodiment of the invention isused to create 32 small electron beamlets 220 focused on the wafer 242for exposure of electron sensitive resist. The 32 beamlets can beblanked and raster scanned over the wafer in order to produce alithographic pattern within the resist corresponding to the desired ICchip pattern. Along with stage motion, each column writes patternswithin a 20 mm×20 mm column footprint area 100 on the wafer 220. With201 columns simultaneously writing on the wafer 242, the time requiredto pattern the entire wafer is equivalent to the time required for onecolumn to pattern its own 20×20 mm footprint area 100. The detailedoperation of each electron optical component is described in theparagraphs below. In other embodiments of the invention, the number ofcolumns and the number of beams per column can be altered. However, theproduct of the number of columns and the number of beams per columnshould be in the range of 4,000 to 12,000.

[0044] WRITING STRATEGY—From FIG. 1, it can be seen that for a 300 mmwafer, a total of 201 columns simultaneously write on the surface of thewafer 242. Each column covers an approximately 20 mm×20 mm squarefootprint 100 on the surface of the wafer. Thus, the entire surface areaof the wafer 242 is covered by the 201 columns. Within each column thereare 32 beams 220, produced by 32 field emission electron sources 202 inthe column (described below). The 32 beams are 1.6 μm apart from eachother on the wafer 242, and are simultaneously scanned along theX-direction (as shown in the axes 510 in FIG. 5). Each pixel is 25 nm×25nm, and deflectors scan 64 pixels on the wafer to create a 1.6 μm-widesubstripe (as shown in FIG. 4e). Since the beamlets 220 begin the scanon 1.6 μm centers, and all the beamlets scan the 1.6 μm substripessimultaneously, this creates a solid line on the wafer 242 that is 1pixel wide and 51.2 μm long.

[0045] The writing strategy incorporating stage motion is shownschematically in FIGS. 4a-4 e. The schematic diagrams in FIGS. 4a and 4b show the wafer 242, the column footprint 402 for each column, the 32beams 406 within each column, and the stripes 414 written by the 32beams. FIGS. 4c-4 e show increasing magnification of the stripe 414written by 32 beams 406 and the substripe 410 written by a single beam.The stripe 414 is the area scanned by all 32 beams 406 and extendsacross the whole length of the column footprint 402. The substripe 410is the area scanned by a single beam and extends across the whole lengthof the column footprint 402. Thus, the stripe 414 is 51.2 μlm inwidth×20 mm long, and is composed of 32 substripes 410, which are 1.6 μmin width×20 mm long. The width of each substripe 410 is composed of 64writing pixels, each of which are 25 nm×25 nm. FIG. 4c shows a view of asingle stripe 414, indicating the 32 individual substripes 410. FIG. 4dis a magnified view of FIG. 4c, and shows how each substripe 410 iscomposed of 64 writing pixels. FIG. 4e is a magnified view of FIG. 4d,showing the beam retrace during the writing of each substripe width.

[0046]FIG. 5, which shows a schematic 4×4 array of columns, each with 4out of the 32 tips shown, is used to illustrate writing strategy and thesynchronized stage motion. The individual tips 502 (4 out of 32),electron gun 504, column diameter 506, and X-Y axes 510 are shown. Ascan be seen, the tips 502 from all of the columns are aligned along theX-direction. Thus, the linear array of tips forms a linear array ofspots on the wafer. The beams are scanned in the X-direction to fill inthe area between adjacent spots on the wafer. The stage is scanned backand forth in the Y-direction. Referring back to FIG. 4, as the stagemoves, the 32 beams 406 are simultaneously scanned in the X-direction tocreate a stripe 414 that is written across the 20 mm column footprint402. As the stage scans across the entire footprint 402 of the column,the resulting cell stripe is 20 mm long and 51.2 μm wide that is drawnin the Y-direction corresponding to the stage motion. This can bethought of as a paint brush. As each column has 32 beams, then eachcolumn “paints” its own stripe 414. After completing one 20 mm passacross the column footprint, the stage steps the wafer 51.2 μm in theX-direction, and travels back across the column footprint 402 in thedirection opposite to its first pass, as depicted in FIG. 4b. The columnfootprint 100 can also be seen schematically in FIG. 1. This process isrepeated until the entire 20 mm×20 mm column footprint 402 is writtenwith approximately 400 stripes 414. The wafer stage motion is called aserpentine motion (back-and-forth, writing both ways), covering the 20mm square column footprint 402 with about 400 stripes 414 over a periodof roughly 90 seconds. Since all of the columns are writing at the sametime, this is also the time that it takes to write the whole wafer 242.

[0047] ELECTRON GUN—The position of the electron gun 250 within theoptical column can be seen in FIG. 2. In standard terms, this isconsidered the “top” of the column (at the left of FIG. 2), and thewafer 242 is located at the “bottom” of the column (at the right of FIG.2). Each column has its own electron gun 250 that consists of twoprimary components: the electron source and the aperture-deflector. In apreferred embodiment of the invention, both components aremicrofabricated on their own die, then flip-chip bonded together.

[0048]FIG. 6 is a schematic of a single emitter and a singleaperture-deflector element within the electron gun 250, showing thesource substrate 204, field emitting tip 202, gate electrode 206, focuslens electrode 208, focus shield electrode 210, spray aperture 212,alignment deflectors 214, object aperture plate 216, blanking electrode218, object aperture 602, tip current regulation circuit 608, currentcollection area 606 of the object aperture plate, and aperture-deflectorassembly 610. An electron gun 250 in the preferred embodiment of theinvention consists of 32 microfabricated Spindt-type field emittersarranged in a line with a center-to-center spacing of approximately 100μm. The Spindt cathodes are fabricated on silicon wafers usingwell-known techniques [I. Brodie and P. Schwoebel, IEEE Proc., vol. 82,no. 7 pp. 1006 (1994)]. Various focusing 208 and shielding electrodes210 may be added to the standard gated Spindt emitter, as discussed inU.S. Pat. Nos. 5,430,347 and 5,637,951. In one embodiment, eachindividual emitter has its own independently addressable gate 206 andfocus electrodes 208. A voltage difference on the order of 100 V for agate hole diameter of 1 μm is applied between the field emitting tip 202(electrically connected to the source substrate 204) and the gateelectrode 206 in order to extract electrons from the tip 202 into thevacuum by field emission. The focus lens electrode 208 is held near thepotential of the tip 202 and focuses the resulting electron emissioninto a parallel beam. Typically, the potential applied to the focuselectrode 208 must be a function of the potential on the gate electrode206. The lens shield electrode 210 is used to eliminate crosstalkbetween adjacent electron beams 220, in case their operating voltagesdiffer significantly. The lens shield electrode 210 could be continuousfor the whole array of 32 emitters, or could be discontinuous, providedthat all parts are held at the same potential. The lens shield electrode210 is typically held at the average potential of the individual focuslens electrode potentials. These four elements—the field emitting tip202, the gate electrode 206, the focus lens electrode 208, and the lensshield electrode 210—are all fabricated on the same source substrate204.

[0049] In a preferred embodiment of the invention, the field emittingtips 202 (see FIG. 2) are held at electrical ground and the wafer 242 isheld at high voltage (50-120 kV). High voltage is required because theelectrons should have very high energy in order to reduce scatteringwithin the resist on the wafer 242. Such scattering will blur theexposure and degrade writing resolution. In another embodiment, thewafer 242 is held at ground and the tips 202 are held at high (negative)voltage.

[0050] Another consideration is the minimization of capacitance of themulti-electrode cathode structure. A high capacitance is undesirable dueto the RC time constant and stored energy (a large time constant willlimit the frequency and rise time for the cathode's driving voltage, anda large stored energy can increase the occurrence of cathode failuresdue to arcing). The capacitance can be minimized by keeping the overlapand extent of electrodes to a minimum that is still consistent withtheir electron optical function.

[0051] Other embodiments include other types of microfabricated electronsources that can be integrated with the aperture-deflector 610 (see FIG.6) and be suitable for the electron optical system described herein.Examples of these sources include, but are not limited to, singlecrystal silicon field emitter microcathodes with or without an emissionenhancing coating on the silicon surface (e.g., metal, carbide or metalsilicide coatings); and single crystal tungsten field emitters etchedfrom a single crystal tungsten substrate—tungsten is attractive becauseit is the most fully characterized and best performing cathode known inthe art of single crystal emitters used in field emission electronmicroscopes. It is considered that a charged particle optical system asdescribed herein could be designed to work with ions. It may be possiblethat a microfabricated ion source can be integrated with theaperture-deflector 610, in a way that is similar to the descriptionabove for the electron source.

[0052] Referring to FIG. 6, after the electron beam is extracted fromthe tip 202 and focused into a parallel beam by the focus lens electrode208, the electrons travel 100 to 500 μm and pass through theaperture-deflector assembly 610. The aperture-deflector assembly 610 forall 32 tips (only one of which is shown in FIG. 6) is fabricated on asingle substrate and consists of the spray aperture 212, the alignmentdeflectors 214, the object aperture plate 216, and the blankingelectrodes 218. The function of the spray aperture 212 is to provide auniform accelerating region (between the lens shield electrode 210 andthe spray aperture 212) and to block out all spherically aberrated beamsfrom passing through the alignment deflectors 214. The sphericallyaberrated beams will not travel through the column with the correcttrajectory and should not be allowed to pass through the object aperture602.

[0053] The remaining electrons not blocked by the spray aperture 212travel through the alignment deflectors 214 and to the object apertureplate 216, as shown in FIG. 6. The purpose of the alignment deflectors214 is to steer the electron beams 220 into the blanking aperture 234 atthe bottom of the electron optical column (see FIG. 2). This blankingaperture 234 is positioned at the back focal plane of the immersion lens238, thereby minimizing aberrations. This will be discussed later. Thealignment deflectors 214 shown in FIG. 2 and FIG. 6 actually representan array of 32 alignment deflectors for each electron gun. There is onealignment deflector 214 for each field emission tip 202. The array ofalignment deflectors 214 is a key feature of the electron gun 250because it allows for precise positioning of a large number of parallelelectron beams.

[0054] The alignment deflectors 214 are a set of non-equisectordodecapole deflectors, as shown in FIG. 7. There are 12 poles(electrodes) for deflection, but the voltages on these poles are appliedsuch that only 4 individual voltages are required. FIG. 7 shows theelectrode arcs 702, voltages supplies 704, 706, 712, & 714, theelectrical interconnects 708 between the electrodes, and the fieldslines 710 formed within the beam region. The electrode arcs 702 havediffering lengths. The electrode interconnects 708 are fabricated suchthat there are four sets of 3 electrodes. The interconnects can eitherbe patterned directly onto the device, or can be connected externally.Each set of 3 electrodes is tied to a different voltage supply 704, 706,712 & 714. Using the connections shown in FIG. 7, the non-equisectordodeacpole deflector deflects an electron beam with significantlyreduced aberrations compared to a typical quadrupole [X-R. Jiang andZ-F. Na, J. Vac. Sci. Technol. B 5(1) (1987)]. The aberrationcoefficients are comparable to an octupole design, but a non-equisectordodecapole requires only 4 individual voltages 704, 706, 712 & 714rather than 8 individual voltages. This significantly reduces the numberof interconnects required in the aperture-deflector assembly 610. Thedrawback to using a dodecapole structure rather than an octupolestructure is that the bore diameter should be somewhat larger in orderto allow the formation of a uniform electric field 710 within the beamregion. Also, the fabrication of the deflector electrodes 702 should bemore precise in terms of mechanical tolerance. In the presentembodiment, the inner diameter of the spray aperture 212 is 12 μm, theinner diameter of the dodecapole deflectors is 23 μm, the length of thedodecapole deflectors is 5 μm, and the size of the object aperture 602is 1.5 μm×1.5 μm. The combination of the individual tips 202, focusingelectrodes 208, object apertures 602 and alignment deflectors 214 can bethought of as a “perfect lens”. A perfect lens takes an incomingparallel beam and focuses the beam to a single point. The electron gun250 creates a linear array of individual, parallel electron beams thatcan be independently focused to a single point (the blanking aperture234, in this case), thus simulating a perfect lens.

[0055] In other embodiments of the alignment deflector, the alignmentdeflector 214 can be a quadrupole or an octupole. The quadrupoleconfiguration has the same number of interconnects as the dodecapoleconfiguration, and is easier to fabricate, but may introduce significantaberrations that increase the spot size at the wafer. The octupoleconfiguration has equivalent aberration effects compared to thedodecapole, but requires double the number of interconnects and powersupplies.

[0056] Referring to FIG. 6, the object aperture plate 216 serves twopurposes. First, the object aperture plate 216 is used as a currentsense to regulate the current within the writing beamlet 220. This tipcurrent regulation circuit 608 is shown schematically in FIG. 6. Each ofthe 32 tips 202 and focus lens 208 elements are individually addressablefor the purpose of beam current regulation. The regulation circuit 608,which has a bandwidth of approximately 1 MHz, adjusts the voltage of thegate electrode 206 to control the current measured at the current senseplate 216. This sense plate 216 is positioned around the object aperture802 for accurate determination of beam current passing down the column.This regulation circuit 608 has two additional functions: it can be usedfor high speed proximity effect correction by varying the programmedcurrent, and is also used for noise reduction of the cold field emitter.

[0057] The second function of the object aperture plate 216, as shown inFIG. 6, is to define the object aperture 602, through which theelectrons travel. All of the electrons passing through the objectaperture 602 will hit the wafer 242 unless the beamlet 202 has beenblanked. The optics of the electron optical column is such that theobject aperture 602 is imaged on the wafer 242 with a demagnificationfactor. In the preferred embodiment of the invention, the objectaperture 602 is 1.5×1.5 μm in size and the demagnification factor is{fraction (1/60)}, resulting in a spot size of 25 nm×25 nm on the wafer242, spaced 1.6 μm apart. In other embodiments of the invention, theobject aperture 602 can be square or round, depending primarily on thewriting strategy being employed (circular apertures work best with amulti-pass gray-scale printing technique, assuming that a Gaussian beamprofile is generated; whereas a square aperture 602 will provide morecurrent at the wafer 242 if throughput is to be maximized above otherconsiderations and also provides the advantages of a shaped beam). Inother embodiments, the object aperture 602 can range from 1 to 3 μm indiameter.

[0058]FIG. 8 depicts a schematic representation of the blanking process,showing the electron gun 250, beamlet 220, blanking electrodes 218, lensplate 232, wafer 242, beam location during writing 802, blanking fieldlines 804, and beam location during blanking 806. The blankingelectrodes 218 consists of 2 parallel plates that are positionedimmediately after the object aperture. When the potential differencebetween the blanking electrodes 218 is zero, the beamlet 220 is notdeflected and can pass through the blanking aperture 234 and onto thewafer 242, assuming that the alignment deflectors 214 have been set sothat the beam is steered into the blanking aperture. When a smallpotential difference is applied to the blanking electrodes 218, anelectric field with electric field lines 804 forms between the blankingelectrodes 218, and the beamlet 220 is deflected by the electric field.A one volt potential difference is sufficient to deflect the beamlet 220off the blanking aperture 234 and onto the lens plate 232, as shown bythe beam location 806 in the lower part of FIG. 8. The center tip isshown in FIG. 8, as evidenced by a perfectly horizontal beamlet.Off-axis tips at the source offer the same effect, but the beam is bentfrom its object aperture 602 position down to the blanking aperture 234,which is located exactly on the optical axis. The voltage required toblank an off-axis beam is the same as that for the on-axis beam. Becauseof the small voltages involved (1 V), the blanking bandwidth can bequite high. It is estimated that 250 MHz can easily be reached, and thisis sufficient to allow for 30 wafers/hour writing throughput for asingle multi-column, multi-beam writing head. The blanking of thebeamlet 220 is performed in a manner called “conjugate blanking”. Thismeans that as the beamlet 220 is deflected by the blanking electrodes218, the spot on the wafer 242 does not move. Conjugate blanking isdesirable so that the image on the wafer 242 is not blurred during theblanking process. The reason that conjugate blanking has been achievedin this system is because the optics are imaging the object apertures602 on the wafer, and the blankers 218 are at essentially the sameposition. Therefore, the actual image position does not change.

[0059] In a preferred embodiment of the invention, the blanking of thebeamlet 220 is performed in a gated manner. The plots of the blankingsignal 904, gating signal 902, blanking field 906, and beam current 908versus time are shown schematically in FIG. 9. A constant (periodic)square wave gating signal 902 is applied to one of the two electrodesthat make up a set of blanking electrodes 218. This square wave has afrequency equal to the writing (or data) frequency, where the beamlet220 is unblanked for only those portions of the gating signal 902 wherethe gating voltage is 0 V (about half of the gate period, as shown).This allows the beamlet 220 to pass through the blanking aperture 234for only half of the time. The pattern data is then applied to theblanking signal 904, which is the potential applied to the secondelectrode in the set of blanking electrodes. The blanking field 906 isthe magnitude of the electric field with electric field lines 804 (asshown in FIG. 8) between the blanking electrodes 218, and the beamlet220 is unblanked only when blanking field 906 is at zero. The beamcurrent 908 is the current that is writing on the wafer 242. As can beseen, the beam current 908 matches the applied blanking signal 904,indicating that the pattern data that is input into the blanking signal904 is being correctly written on the wafer 242. The benefit of thissystem is that the rise and fall times of the blanking signal 904 do notsignificantly affect the beam deflection because they occur during an“off” state of the gating signal 902. The rise and fall times of thegating signal 902 can be made very small because the gating signal is asteady-state square wave signal. The disadvantage of this system is thatthe beamlet current 908 is only on the wafer 242 for half of the time,even when the pattern data requires an unblanked pixel. Therefore, inorder to achieve the same writing dose, the beamlet current 908 must bedouble that of the typical writing current. In the previous calculation,a beamlet current of 12 nA was required for a throughput of 30 wafers/hrper writing head. Using this gated blanking system this beamlet currentmust be doubled to 24 nA.

[0060] The fabrication of the aperture-deflector assembly is shownschematically in FIGS. 10a-10 o. The process steps of this fabricationsequence are described below. Two alternative methods are described—theprincipal difference being the starting substrate.

[0061] Method A

[0062] 1. Start with SOI (silicon on insulator) wafer—300 micron thickwafer 1002 with up to 2 μm of oxide 1004 on one surface, capped with 5μm of Si 1006.

[0063] 2. Grow 1 μm of thermal oxide or deposit 1 μm CVD oxide 1008 onboth sides of wafer. FIG. 10a.

[0064] 3. Lithography on topside followed by dry etch (RIE) throughoxide 1004 and Si 1006 layers, defining a 12 μm diameter circular recess1010 with a post 1012 in the center (the post 1012 serves to planarizethe structure for future process steps). FIG. 10b.

[0065] 4. Deposit 0.2 μm of silicon nitride 1014 on the backside;lithography on backside followed by dry etch (RIE) of nitride 1014. FIG.10c.

[0066] 5. Lithography on topside followed by evaporation of 40 nm of Crfollowed by 0.5 μm of Au 1016. Liftoff excess metal. FIG. 10d.

[0067] 6. Deposit (PECVD) a total of 5 to 8 μm of low stressSiO_(x)N_(y) 1018 on the topside. FIG. 10e.

[0068] 7. Lithography on topside followed by dry etch (RIE) through theSiO_(x)N_(y), 1018 exposing the Au 1016. FIG. 10f.

[0069] 8. Electroplate 5 to 8 μm of Au 1020 onto the exposed evaporatedAu surface. FIG. 10g.

[0070] 9. Deposit (PECVD) 1 μm of SiO_(x)N_(y) 1022 onto the topside,followed by 0.3 μm of TiW 1024. FIG. 10h.

[0071] 10. Lithography on topside followed by dry etch (RIE) of TiW1024. This defines the object aperture 602; in different embodimentsthis aperture 602 may be 1.5 or 3.0 μm in diameter. FIG. 10i.

[0072] 11. Deposit (PECVD) 1 μm of SiO_(x)N_(y) 1028 on topside. FIG.10j.

[0073] 12. Evaporate or sputter (the latter provides better filmcontinuity over steps) 1 to 1.5 μm of Al 1030 on topside. Lithography ofthe blanking electrodes 1032 on topside followed by wet etch of Al. FIG.10k.

[0074] 13. Lithography on topside followed by dry etch (RIE) ofSiO_(x)N_(y) 1028, exposing TiW 1024 layer. FIG. 101.

[0075] 14. Protect topside with photoresist (wafer may also be attachedby wax to a glass plate); lithography on backside, followed by an etchwith BOE (buffered oxide etch)—etching through the thermaloxide—followed by a KOH etch—etching crystallographically into the Siwafer 1034. FIG. 10m.

[0076] 15. Strip photoresist from entire structure. Protect topside withphotoresist and etch exposed backside thermal oxide with BOE, followedby a KOH etch—etching crystallographically into the Si wafer until thethermal oxide 1004 on the topside of the wafer is reached; alternativelya timed KOH can be followed by a dry (RIE) etch to reach the oxide 1004.FIG. 10n. Note: the Si post 1012 drops out during this step.

[0077] 16. Continue with a BOE etch—etching the thermal oxide 1004 andSiO_(x)N_(y) 1018 from the backside. FIG. 10o.

[0078] Method B

[0079] 1. Start with a 300 μm thick Si wafer.

[0080] 2. Grow 1 μm of thermal oxide or deposit 1 μm CVD oxide on bothsides of wafer.

[0081] 3. Lithography on topside followed by dry etch (RIE) through theoxide and approx. 10-20 μm into the Si, defining a 12 μm diametercircular recess with a post in the center.

[0082] 4. Deposit 0.2 μm of silicon nitride on the backside; lithographyon backside followed by dry etch (RIE) of nitride.

[0083] 5. Lithography on topside followed by evaporation of 40 nm of Crfollowed by 0.5 μm of Au. Liftoff excess metal.

[0084] 6. Deposit (PECVD) a total of 5 to 8 μm of low stressSiO_(x)N_(y) on the topside.

[0085] 7. Lithography on topside followed by dry etch (RIE) through theSiO_(x)N_(y), exposing the Au.

[0086] 8. Electroplate 5 to 8 μm of Au onto the exposed evaporated Ausurface.

[0087] 9. Deposit (PECVD) 1 μm of SiO_(x)N_(y) onto the topside,followed by 0.3 μm of TiW.

[0088] 10. Lithography on topside followed by dry etch (RIE) of TiW.This defines the object aperture; in different embodiments this aperturemay be 1.5 or 3.0 μm in diameter.

[0089] 11. Deposit (PECVD) 1 μm of SiO_(x)N_(y) on topside.

[0090] 12. Evaporate or sputter 1 to 1.5 μm of Al on topside.Lithography on topside followed by wet etch of Al. This defines theblanking electrodes.

[0091] 13. Lithography on topside followed by dry etch (RIE) ofSiO_(x)N_(y), exposing TiW layer.

[0092] 14. Protect topside with photoresist (wafer may also be attachedby wax to a glass plate); lithography on backside, followed by an etchwith BOE—etching through the thermal oxide—followed by a KOHetch—etching crystallographically into the Si wafer.

[0093] 15. Strip photoresist from entire structure. Protect topside withphotoresist and etch exposed backside thermal oxide with BOE, followedby a timed KOH etch—etching crystallographically into the Si wafer,followed by a dry (RIE) etch to reach the circular recess that wasetched into the top surface at step 3 above.

[0094] 16. Continue with a BOE etch—etching the SiO_(x)N_(y) from thebackside. FIG. 11.

[0095]FIG. 11 shows a schematic of a completed aperture-deflector usingMethod B, indicating the aperture substrate 1102, alignment deflectors1104, object aperture plate 1106, object aperture 1108 and blankingelectrodes 1110.

[0096]FIG. 12 schematically shows how adjacent aperture-deflectorassemblies appear within the aperture-deflector device, indicating thesubstrate 1202, spray aperture 1204, alignment deflectors 1206, objectaperture plate 1208, and blanking electrodes 1210. Only 3 out of 32aperture-deflectors are shown. Each assembly is capable of individuallydeflecting an electron beam using the alignment deflectors 1206 towardsthe blanking aperture 234 (see FIG. 2), and provides an individualcurrent sense (object aperture plate 1208) for source currentregulation. Individual blanking is also provided by the blankingelectrodes 1210. The details of the traces and contacts to the differentelectrodes are not shown. Various methods for laying out traces andmaking contacts, including the use of vias, would be familiar to anyoneskilled in the art of semiconductor device fabrication. As can be seenin FIG. 12, the silicon substrate 1202 is etched beyond theaperture-deflector assemblies on the edges. Calculations indicate thatthis will not significantly affect the electron trajectories, althoughthe spacing between edge of the device and the place where the substrateis etched should be as large as possible without compromisingfabrication yield. A typical aperture-deflector substrate 1202 is 300 μmin thickness, and this represents the spacing between the field emissiontip 202 (shown in FIG. 2) and the spray aperture 1204.

[0097]FIG. 13 is a schematic representation of the overlay of theaperture deflector die and the source die, showing the position of thegold bonds 1302, the aperture-deflector die 1304, source die 1306 andthe silicon oxynitride pedestals 1308. The gold bonds 1302 are requiredon the source die 1306 to make a strong eutectic bond to the backside ofthe aperture deflector wafer 1304. The silicon oxynitride pedestals 1308are required for electrical standoff between the two devices to preventelectrical shorting to the traces on the source die 1306. Note from FIG.13 that the source die 1306 is larger than the aperture-deflector die1304; this is to allow for ease of making electrical contacts tocontact/bond pads on the top peripheral surface of the source die 1306.Electrical contacts are made to the top surface of theaperture-deflector die 1304.

[0098]FIG. 14. schematically represents the entire electron gun afterflip chip bonding, showing the source die substrate 1402,aperture-deflector substrate 1404, gold-silicon eutectic bond 1406,field emission tips 1408, gate electrodes 1410, focus electrodes 1412,lens shield electrodes 1414, spray apertures 1416, alignment deflectors1418, object aperture plates 1420, object apertures 1422, blankingelectrodes 1424, dielectrics 1426 and pumping aperture 1428. Only 3 outof 32 sources are shown. The aperture-deflectors 610 (see FIG. 6) alignwith the field emission tips 1408 in this device so that the focusedelectron beam travels through the alignment deflectors 1418, objectapertures 1422 and blanking electrodes 1424. Alignment tolerance istypically several μm. A pumping aperture 1428 is required in order tomaintain a high quality vacuum environment at the field emission tips1408. This allows improved performance and stability. The entireelectron gun 250 (see FIG. 6) after flip-chip bonding of the twosubstrates is roughly 10 mm square. The electron gun will easily fitwithin the 20 mm square column footprint 100, while allowing therequired interconnects. This electron gun 250 is typically brazed andwirebonded onto a ceramic header in order to conveniently make all therequired electrical connections to the electron optical components. Notethat in some embodiments, it may be preferred to reduce the spacingbetween the spray apertures 1416 and emitter tips 1408, in which caseone of more of the following may be done: (1) thinner silicon may beused as a starting substrate 1404 for the aperture-deflector assembly610, (2) the fabrication process may be modified to include thinning ofthe aperture-deflector substrate 1404 from the back at a later stage inthe process flow.

[0099] Another embodiment of the aperture-deflector die includes a twodimensional array of aperture-deflector assemblies 610. As with thelinear array, either discrete or large illumination area electronsources could be fabricated on one substrate and the aperture-deflector610 could be fabricated onto a separate substrate that is flip-chipbonded onto the source substrate. Some lithography or imagingapplications may benefit from writing a 2D array on the wafer ratherthan a linear array.

[0100] Another embodiment of the electron gun 250 uses a floodillumination source rather than discrete electron sources. In thisembodiment, the part of the beam that passes through the object aperture1422 can be individually controlled by the alignment deflectors 1418 togive a multiplicity of beamlets 220 traveling down the column.

[0101] ACCELERATION REGION AND DEFLECTORS—The accelerating and scanningdeflector regions of the electron optical column represents the vastmajority of the length of the column, as shown in FIG. 2, which isroughly 160 mm in a preferred embodiment of the invention. Due primarilyto the small scale of the column, all of the lenses, rotators,deflectors, blankers, etc. are electrostatic; no magnetic opticalelements are used. Concerns with magnetic elements are the complexitiesof the fabrication on such a small scale and magnetic screening of onecolumn from the next in the closely packed array of columns. Most of thecolumn components are precision-machined metals, insulating ceramics andconductive ceramics. Some of the more complex metal electrodes arescreen printed onto ceramic; simpler electrodes maybe be brazed toceramic. Standard mechanical and optical alignment techniques areutilized to ensure that all components are properly situated.

[0102] Referring to FIG. 2, after passing through the object aperture602 and the blanking electrodes 218, the beamlet 220 enters theaccelerating region. In the preferred embodiment of the invention, theelectrons are accelerated from 100 eV to approximately 6000 eV, wherethe scanning deflectors are located. The acceleration can beaccomplished using simple plates 224 on the order of 25 to 250 μm thickwith holes corresponding to the accelerating column bore. These platesare typically metal, and can be made from beryllium copper or any othernon-magnetic material. Typical column bores are 10 mm. The appliedpotential of each plate 224 would increase linearly from the 100 V tothe 6000 V potential. The accelerating column can also be made fromresistive ceramic in one piece. A linearly increasing potential isdesirable because it does not introduce lensing effects in the beam thatcould distort the beam shape. In another embodiment the plates can bereplaced with mesh grids. In the preferred embodiment, each column doesnot have an individual accelerating region, but the accelerating regionof all 201 columns of the writing head are combined into one unit. Thisunit can have plates 224 extending across the whole area of the wafer,with holes for the individual columns.

[0103]FIG. 15 is a schematic representation of the Rotator 222 (see FIG.2) and its effect on the beamlet, showing the octupole deflectorelectrodes 1502, source rotation angle 1504, force vectors on thebeamlets 1506, actual object aperture locations 1508, virtual objectaperture locations 1510, and final trajectories of the beamlets 1512.Only 5 out of 32 beamlets are shown. The Rotator is the first electrodeof the accelerating region, and in one embodiment, the electrodes arescreen-printed metallic films on top of a ceramic substrate. The Rotatorserves two purposes. First, the dc potential of the deflector electrodesdefines the beginning of the accelerating region. This voltage isapproximately 200-300 V. Second, the rotator deflector acts as ameridional plane rotator to correct for rotational misalignment of thesource 1504 relative to the rest of the column. This function isnecessary in a multi-column arrangement as mechanical rotationalmisalignments 1504 cannot be corrected using the scan. Normally, amagnetic lens is used to correct for rotation; however, it would beimpractical to use 200+ magnetic lenses. To first order, anelectrostatic quadrupole lens can perform this function as long as thebeam rotation 1504 is not large. This is shown by the voltages appliedto the electrodes in FIG. 15. It can be seen that by applying voltagesto the octupole deflector 1502, force vectors 1506 will push one side ofthe 32 beamlets “up”, while pushing the other side of the 32 beamlets“down”. Assuming the amount of rotation 1504 is small, the force vectors1506 are linear across the linear array of 32 beamlets 220. By bendingthe 32 beamlets 220 as shown in FIG. 15, the virtual object apertures1510 are shifted away from the actual object apertures 1508 and appearas a rotational correction of the mechanical misalignment. Thetrajectories 1512 leaving the rotator make it appear that the objectapertures are at the position of the virtual object apertures 1510.

[0104] As can be seen in FIG. 2, after passing through the acceleratingregion and being accelerated to roughly 6000 eV, the electrons passthrough the shield electrode 226 and the mainfield 228 and subfield 230scanning deflectors. The shield electrode 226 is the last electrode inthe accelerating region and defines the start of the deflector region.It is typically a metal, metallized ceramic or conducting ceramic plate.The voltage on the shield electrode 226 and the dc voltage level on thesubfield deflectors 230 are used to help focus the 32 beamlets 220 onthe wafer 242. Varying the voltages will be used to correct formechanical tolerance in the column length.

[0105]FIG. 16 schematically depicts the operation of the mainfielddeflectors 228, showing the position 1602 of the undeflected beamlet onthe wafer, the electric field lines 1604 formed during deflection, thedisplacement of the virtual object 1606, the deflected position 1608 ofthe beamlet on the wafer, the first octupole deflector 1610 and thesecond octupole deflector 1612. Only 1 beamlet out of 32 is shown inFIG. 16. In the preferred embodiment of the invention, the mainfielddeflectors 228 are a set of double octupole deflectors 1610 & 1612.Therefore, there are 16 total connections to this deflector assembly.The mainfield deflectors 228 are fabricated using Ti alloy electrodesbrazed to ceramic, which is useful for correctly spacing the deflectorfrom the lens plate. Machining of the column bore is done after brazing,so as to ensure concentricity of both sets of octupole deflectors. Themainfield deflector 228 acts on all 32 beamlets 220 simultaneously. Twooctupole deflectors 1610 & 1612 are used in order to ensure that thebeamlets 220 pass through the blanking aperture 234 and aretelecentrically scanned on the wafer 242. Telecentric scanning meansthat the beamlets 220 hit the wafer 242 perpendicular to the wafersurface, and this allows for a larger depth of field of the focused beamat the wafer 242. By using the double octupole design, the firstoctupole 1610 “pushes” the beamlet 220 in one direction, then the secondoctupole 1612 “pulls” the beamlet 220 in the opposite direction.Therefore, all 32 beamlets can pass through the blanking aperture 234 atthe appropriate position and will scan telecentrically on the wafer 242.

[0106] The mainfield deflectors 228 have three function: (1) correct formechanical misalignment of the column assembly, (2) track the stagemotion as it moves the wafer 242 during writing, and (3) perform a largearea scan to find alignment marks. The mainfield deflectors 228 have atotal deflection capability of approximately ±10 μm in both the X and Ydirections on the wafer 242. However, at the edges of this scanfield,the aberrations in the beam are significant, and beam resolution andspot size will be affected. Modeling of beam resolution and spot sizehas been carried out using the commercially available software packagesSIMION 3D, ver. 6.0 and Eric Munro's MEBS. These calculations indicatethat the writing field, within which the spot size is sufficiently smallto expose a single pixel area, is approximately ±5 μm in X and Y for themainfield deflectors 228.

[0107] In its first function, the mainfield deflectors 228 are used tocorrect for X-Y mechanical misalignment of the electron gun 250 withrespect to the rest of the column. This misalignment is unavoidable dueto assembly errors, and the electron gun assembly 250 may be as much as25 μm offset from the rest of the column. Due to the {fraction (1/60)}demagnification of the object apertures 602, this corresponds to lessthan a ±1 μm shift on the wafer 242. Thus, the mainfield deflectors 228will be able to compensate for this assembly error. This error is fixedfor the lifetime of the column.

[0108] In its second function, the mainfield deflectors 228 are used totrack the stage motion during the writing process. A typical stage has aposition accuracy of ±1 μm; however, since the pixel size is only 25 nmsquare, the position of the stage must be known to a much higher degreeof accuracy. This information can be determined, for example, usinglaser triangulators. This spatial location information is fed in advanceto the mainfield deflectors 228, which deflect all 32 beamlets 220 tothe correct location of the wafer 242.

[0109] In its third function, the mainfield deflectors 228 are used tosearch for alignment marks on the wafer 242. This is achieved byscanning the full scan field (±10 μm square) to find the alignmentmarks. Global positioning should be sufficiently accurate to place thealignment marks within this scan field. Although the resolution or spotsize of the beamlet 220 may not be as small as is needed for writing, itwill be sufficient for the location of the alignment marks. Thealignment marks are typically made using heavy atoms so that thebackscattered electron efficiency of the mark is significantly higherthan that of bare silicon (or silicon oxides). Therefore, the BSEdetectors 236 will be able to detect the contrast between the heavyalignment marks and the areas with no marks. Typical alignment markmetals can be, but are not limited to, gold, tungsten, andtitanium-tungsten.

[0110] The bandwidth of the applied voltage signal for the mainfielddeflectors 228 is determined by the desired scan speed in order to findthe alignment marks on the wafer. Typically, a bandwidth of roughly 50kHz is more than sufficient for the mainfield deflectors 228.

[0111]FIG. 17. schematically depicts the operation of the subfielddeflectors 230, showing the position 1702 of an undeflected beam on thewafer, the electric field lines 1704 formed during deflection, thedisplacement of the virtual object 1706, and the deflected position 1708of the beamlet on the wafer. Only 1 beamlet out of 32 is shown in FIG.17. In the preferred embodiment of the invention, the subfield deflector230 is a quadrupole deflector with 4 independent electrodes. An octupoledeflector could be used; but since the deflection range of the subfielddeflector is small (<±1 em), the aberrations introduced by thequadrupole deflector do not significantly affect beam shape at the wafer242. This has been confirmed using SIMION 3D, ver. 6.0 and Eric Munro'sMEBS software package. A quadrupole has fewer connections, and thereforeis preferable from a manufacturing point of view. In another embodimentof the invention, the subfield deflectors 230 could be octupole orhigher order deflectors. The mainfield deflectors 228 are a set ofdouble deflectors 1610 & 1612 in order to maintain telecentric scanning.The subfield deflector 230 only requires a single set of deflectorsbecause the blanking aperture 234 is located very close to the subfielddeflector 230. This allows a single set of deflectors to scan the beamthrough the middle of the blanking aperture 234 and achieve telecentricscanning on the wafer 242. This has also been confirmed using SIMION 3D,ver. 6.0. The subfield deflectors 230 are fabricated similarly to themainfield deflectors 228.

[0112] The subfield deflectors 230, as seen in FIG. 17, serve twopurposes. First, the dc voltage of the deflectors can be adjusted inorder to achieve a small amount of focusing of the 32 beamlets 220 onthe wafer 242. Ideally, if all of the columns were mechanicallyidentical, this would not be necessary. However, since this is notrealistic, the shield electrode 226 and subfield deflector 230 dcpotential values are used to correct for minor adjustments in the focusand demagnification of the electron optical system. The second functionof the subfield deflectors 230 is to scan the 32 beamlets 220 on thewafer 242 during the writing process. When the beamlets 220 hit thewafer 242 at any given time, the spots on the wafer 242 are roughly25×25 nm in size, and are separated by 1.6 μm spacing. The subfielddeflector 230 scans all 32 beamlets simultaneously in the directionparallel to the array axis (in the X-direction on FIG. 4) to fill in thespace between the pixels. Therefore, the subfield deflector 230 onlyrequires scanning of ±0.8 μm from its nominal position, corresponding toa total of 64 pixels between beamlets 220.

[0113] Although the subfield deflector 230 is only required to scan inone direction, a quadrupole deflector is required. There are two reasonsfor this: (1) rotational misalignment of the subfield deflector assemblywith respect to the other parts of the column, and (2) slight changes inthe deflection due to constant stage motion. A quadrupole deflectorallows for a slight angle in the scan to correct for the rotationalmisalignment in the column. Most likely, the rotational misalignmentwill be small. The rotators can also be used to correct for some of thismisalignment. The stage motion, however, dictates the use of aquadrupole deflector for the subfield deflector 230 rather than a simpledipole deflector. The reason for this is illustrated in FIGS. 18a and 18b, which show a schematic of pixel locations on the wafer foruncorrected and corrected stage motion, respectively, including the scan1802 of the beam on the wafer for uncorrected stage motion, scan 1804 ofthe beam on the wafer for corrected stage motion, stage motion 1806,scan direction 1808, substripe width 1810, pixel width 1812, andadjacent substripes 1814. Since the stage motion 1806 is continuous, thedeflection of the beamlet 220 using the subfield deflectors 230 shouldcompensate for this motion 1806. Otherwise, the end pixel of the beamletwill not match up with the beginning pixel of the adjacent substripe1814, as shown in FIG. 18a. A very small correction should be applied tothe deflection 1808 in the Y-direction to compensate for the stagemotion 1806 during the writing of a single line so that the edges of thepixels match up to the adjacent substripe 1814, as shown in FIG. 18b.This correction signal requires at least a quadrupole deflector in orderto fulfill the correction operation.

[0114] The required scan speed of the subfield deflector 230 from FIG.17 can also be calculated. Assuming a blanking rate of 250 MHz and 64pixels per scan, then the time required to scan the beamlets 1.6 μmacross the wafer is 320 ns. Thus, a minimum bandwidth of approximately 3MHz is required. However, in order to simplify the data path going tothe blanking electrodes 218, it is desirable to do a retrace of the 32beamlets 220 rather than writing the data in the reverse direction forevery writing scan. Therefore, in the preferred embodiment of theinvention, the bandwidth of the subfield deflector 230 is considerablyhigher than 3 MHz—typically approaching 100 MHz—in order to achieve afast retrace to the next writing line to maintain a more simplified datasequence to the blanking electrodes 218.

[0115] IMMERSION LENS AND BSE DETECTORS—The bottom part of the columnconsists of the immersion lens 238 and BSE detectors 236, as shown inFIG. 2. The immersion lens 238 consists of the the shape of the backsideof the lens plate 232 and the region above the wafer. By applying highvoltage (50 to 120 kV) on the wafer 242, a high field region is createdbetween the wafer 242 and the lens plate 232 that focuses the 32beamlets 220 onto the wafer 242 into 25×25 nm pixels, which is a{fraction (1/60)} demagnification of the object aperture 602 array inthe electron gun 250. The lens plate 232 is the last column componentabove the wafer. In the preferred embodiment of the invention, the lensplate 232 is common to all of the columns, and houses all of the BSEdetectors 236 for all of the columns. The lens plate 232 is typicallymetal, and can be made of molybdenum, titanium, or any othernon-magnetic material.

[0116]FIGS. 19a and 19 b show a schematic representation of theimmersion lens 238, showing the lens plate 232, wafer 242, blankingaperture 234, immersion field 1902, parallal equipotential lines 1904,curved equipotential lines 1906, centerline of the immersion lens 1906,location 1908 of the on-axis spot on the wafer, location 1910 of theoff-axis spot on the wafer, on-axis beamlet 1912, and off-axis beamlet1914. Only 1 beamlet out of 32 is shown. The lens plate 232 isprecision-machined from a Ti alloy or Mo plate. In the preferredembodiment of the invention, the columns are positioned very accuratelyrelative to each other by mounting all of them to a singleprecision-machined lens plate 232. The shape of the backside of the lensplate is important in creating the curved equipotential lines 1906. Theelectric fields corresponding to these equipotential lines 1906 focusthe beam, as shown in FIG. 19, to a small spot on the wafer. Thus, themachining of these counterbore elements must be precise. The blankingaperture 234 is placed at the back focal plane of the immersion lens 238to allow for telecentric scanning on the wafer 242, as shown in FIGS.19a and 19 b by the perpendicular trajectory of the beamlets 220 withrespect to the wafer 242 during scanning. Telecentric scanning of thebeam on the wafer allows for a larger depth of field at the wafer. Notethat the blanking aperture 234 is placed in a field-free region so as toavoid a lens effect at this point in the column. In FIG. 19a, theon-axis beamlet 1912 travels through the blanking aperture 234 and isfocused onto the on-axis position 1908 on the wafer 242. In FIG. 19b,the off-axis beamlet 1914 enters the blanking aperture 234 at a slightangle, and is focused on the wafer 242 at a slightly offset position1910 compared to the on-axis beam position 1908. The beamlet 1914 inFIG. 19b could be one of several different types of beamlets: (1) anundeflected off-axis beamlet, (2) a deflected on-axis beamlet, or (3) adeflected off-axis beamlet. In all three of these cases, the beamlet1914 will travel through the middle of the blanking aperture 234 and betelecentrically focused onto the wafer 242 at a position 1910 slightlyoffset from the on-axis beam position 1908.

[0117] In one embodiment of the invention, the distance between the lensplate 232 and the wafer 242 is roughly 10 mm. With a total column lengthof approximately 160 mm, this creates a {fraction(1/60)}×demagnification of the object apertures 602 in the electron gun250. In a further embodiment of the invention, the distance between thelens plate 232 and the wafer 242 is roughly 20 mm. With the same columnlength, this creates a {fraction (1/30)}×demagnification of the objectapertures 602. Differing demagnifications can be applied to differentwriting strategies, such as writing with shaped beams or gray scalewriting. For example, a {fraction (1/30)}×demagnification with a roundobject aperture can be used to create a 50 nm Gaussian beam that can beused for a multi-pass, gray scale writing strategy.

[0118]FIG. 20 shows a bottom plan view of a column array, showing theBSE detectors 236 within their housing in the lens plate 232 and thecounterbore 2002 that bends the field lines in the immersion lens. Only4×4 columns are shown out of the 201 columns within a writing head. Asthe beamlets 220 travel through the column and hit the wafer 242, asshown in FIG. 2, they create secondary and backscattered electrons 240that are emitted from the wafer surface. Due to the high immersionfield, the secondary electrons return back to the wafer. However, manyback-scattered electrons 240 will have sufficient energy to returnthrough the immersion field and land on the BSE detectors 236. The BSEdetectors 236 are housed in the bottom of the lens plate 232, and a highvoltage bias (e.g., 1000 V) can be applied to ensure that thebackscattered electrons 240 (see FIG. 2) reach the detectors 236 oncethey enter this housing. The opening in the lens plate 232 needs to bedesigned to allow maximum collection efficiency while shielding thedetector 236 from the electric field of the immersion lens 238 (such adesign may be similar to a wagon wheel in appearance—a circular openingwith thin radial spokes, where care is taken to ensure that all edgesare well-rounded so as not to generate high local electric fields). TheBSE detectors 236 are an important feature in the invention because theywill allow imaging of the wafer surface. This is particularly useful foralignment mark detection and global alignment of the wafer 242 withrespect to the writing head. Because different materials have differentbackscattering coefficients, as the beamlets 220 are scanned on thewafer 242, the amount of backscattered electrons 240 reaching the BSEdetectors 236 changes depending on the material that is hit by theelectron beam. Typically during this scanning mode, only one beamlet 220is used. By scanning the mainfield deflectors 228, an image of the wafersurface can be obtained using the BSE detectors 236, and alignment marklocations can be determined.

[0119] In the preferred embodiment of the invention, four individuallycontrolled BSE detectors 236 are used, as shown in FIG. 20. Fourindividual BSE detectors 236 allow for collection of topographicalinformation from the wafer surface. The BSE detectors 236 may include,but are not limited to, standard silicon detectors (optimized forelectron collection).

[0120] The electron optical system is operated in a vacuum chamber witha pressure of at most 1 E-06 Torr, and preferably 1 E-08 Torr or less.The field emission electron sources are operated at a pressure of atmost 1E-09 Torr and preferably 1E-12 Torr. The apertures in the column,such as the spray aperture 212 and particularly the object aperture 602shown in FIG. 6 will act as differential pumping apertures, facilitatingthe maintenance of a better vacuum in the region of the electronsources.

[0121] CONTROL ELECTRONICS AND ERROR ANALYSIS—The control electronicsrequired to operate each electron optical column for the multi-column,multi-beam lithography system is illustrated in block diagram format inFIG. 21. The electron gun 250, field emission tip 202, source substrate204, gate electrode 206, focus electrode 208, source control 2102,alignment deflector 214, alignment deflector control 2104, objectaperture plate 216, blankers 218, blanker drivers 2106, rotator 222,rotator control 2108, shield electrode 226, shield electrode control2110, mainfield deflectors 228, mainfield deflector control 2112,subfield deflector 230, subfield deflector control 2114, lens plate 232,blanking aperture current sense control 2116, BSE detectors 236 andwafer 242 are shown. The paragraphs below describe the controlelectronics and error analysis for the preferred embodiment of theinvention.

[0122] Even if errors due to incorrect data being input into theblanking electrodes 218, or errors due to current fluctuations from thefield emission tip, are zero, the possibility of writing errors due tooperational failures or calibration errors in one of more of the 201writing columns must be considered. The possible error sources in thecolumn are as follows: the column components themselves, the columninterconnects, and the column drive electronics. It is expected that thecolumn drive electronics will be the most likely source of errors. Thegoal is to achieve a mean time between failures (MTBF) of over 10 yearsfor the entire in-vacuum 201 column array.

[0123] The column drive electronics will consist mostly of analog andmixed signal D/A and A/D devices. As much of this electronics aspossible will be located outside the vacuum environment in easilyreplaceable field-replaceable units (FRUs). However, the bulk of thecritical column drive electronics will need to be located in-vacuumclose to the column components to reduce the interconnect andfeedthrough complexity. Most of the column data, control, and senselines that enter the vacuum chamber will be multiplexed digital signalswith error-correcting code (ECC) as required. It will be critical tohave very high reliability of the in-vacuum components. The vacuumchamber provides a hermetic seal that should help reduce failures.

[0124] Published failures-in-time (FITs, where 1 FIT=1 failure/10⁹ hrsoperation) numbers for analog and mixed signal devices vary widelydepending upon the technology used. For the following calculations, amature CMOS process has been assumed; CMOS is required to control powerdissipation in the vacuum electronics. FIT numbers for these types ofdevices are around 5. TABLE 1 describes a rough estimate of the devicecount for the in-line vacuum drive electronics required for a singlecolumn. The calculations show an MTBF of approximately 6.3 years for theentire lithography writing head. A high level of integration (customASICs) is assumed. TABLE 1 Internal device count and MTBF InternalDevice Count Sub-System per Column Source control 4 Alignment deflector4 control Blanker drivers 1 Rotator control 1 Shield electrode and HV 1control Mainfield deflector control 2 Subfield deflector control 1Current sensors 2 BSE detector 2 TOTAL 18 FIT @ 5/device 90 MTBF/column(years) 1268 MTBF/column array (years) 6.3

[0125] The column can be subject to soft failures that can cause randomwafer errors. These soft failures are “analog” in nature (data errorrates can be made vanishingly small) and might be caused, for example,by a gain or offset drift in one of the deflection drivers. Theself-detection of both hard and soft failures in the column array is acritical design requirement. The multi-column, multi-beam lithographytool will self-test the functionality of all columns, eithercontinuously or at frequent intervals, to detect these errors andprevent yield loss. The paragraphs below will focus on how theself-testing operation can be performed. In this discussion, theinterval between the self-test events is the shortest possible intervalthat can be achieved and still maintain full lithography systemthroughput.

[0126] The performance of the source can be monitored in three differentways: (1) The source stability is monitored continuously via the objectaperture current sensors and source control electronics 2102. (2) Duringthe wafer exchange time (every 120 sec), the source current can bemonitored by unblanking all of the beamlets except the one of interest,and then measuring the current on the blanking aperture. The 32measurements per column can be performed in parallel in all 201 columns,thus the overhead time can be kept to a few seconds (done in parallelwith other tasks). (3) Once each hour or so, a Faraday cup calibrationsubstrate is placed under the column array and the true wafer currentfor each beamlet is measured. The setpoint for the object aperturecurrent servo is re-adjusted, if necessary, to the desired beamletcurrent.

[0127] The alignment deflector control 2104 provides dc deflectionvoltages to the alignment deflector plates. These deflection voltagessteer the individual beamlets 220 into the blanking aperture 234. Afailure in this controller 2104 will be detected within one wafer writetime by Test (2) from above. Once each hour, the alignment deflector 218operation will be completely verified and the setting re-adjusted sothat the beamlets 220 are well-centered on the blanking aperture 234.This will be accomplished by sweeping the alignment deflector platevoltages in a raster scan over the blanking aperture 234 while measuringthe blanking aperture current.

[0128] The blanker drivers 2106 are a set of 32 buffer gates between thedata path and the physical blanking plates. The buffer gates are enabledat a bandwidth of approximately 200-250 MHz. Their operation is verifiedonce each wafer write time by Test (2) from above. More sophisticatedtests that measure the blanking plate rise time and fall time usingstroboscopic techniques can be performed at less frequent intervals.

[0129] The rotator control 2108 provides signals to the rotator 222,which is an electrostatic octupole deflector requiring 8 voltage drivesignals. Drive voltages are set only once during the calibrationprocedure performed during the initial system setup. The rotator settingcan easily be verified at any time as long as an alignment mark ispresent on the wafer or a calibration substrate is mounted on the waferchuck. This verification process involves finding the identicalalignment mark using the two beamlets 220 at the edges of the 32 beamletarray. Any discrepancy in the X-axis mark location represents arotational error in the beamlet array, which can be removed by adjustingthe rotator voltages. This test can be performed on each rotator 222approximately once each hour with no reduction in throughput.

[0130] The Focus and Shield Electrode Control 2110 applies high voltagesignals used to bias the column stack 224 and to provide individualcolumn focusing. These HV signals are common to all columns and areprovided by an external HV supply that is fully instrumented to detectan out-of-tolerance condition. All power supplies in the lithographysystem will be continuously monitored and any out-of-toleranceconditions will be reported to the system controller. The shieldelectrode 226 provides an offset bias voltage for column focusing. Boththe HV supply and the shield electrode supply will be verified once eachhour in a focus check operation that involves loading a calibrationsubstrate onto the wafer chuck. Reference edges on this substrate willbe used to measure the beamlet size and adjust focus. Gross focus errorson the alignment columns would also be detected once each wafer 242during the wafer alignment process.

[0131] The mainfield deflector controller 2112 supplies low frequency(<50 kHz) deflection voltages to the mainfield double octupole deflectorplates 238 to compensate for stage positioning errors and the normalmovement of the writing target under the beamlet 220 caused by stagemovement during scan-line writing times. The verification of themainfield deflectors 228 will be performed once each hour using acalibration substrate. This process involves a series of alignment marklocation operations at different stage position. The alignment marklocation should remain the same at all stage position values. Anysystematic deviation from the ideal mark location as a function of stageposition represents a deflection calibration error. Linear gain androtation errors are easily measured and removed using this method.

[0132] The subfield deflector control 2114 provides 4 voltage signals tothe subfield quadrupole deflector 230. Except for the blanking platedrivers, the subfield deflector 230 is the highest frequency device inthe column. The subfield drivers provide the fast writing rampdeflection voltages to the subfield quadrupole plates. A preferredembodiment uses a single external writing ramp generator, with completegain, offset and linearity monitoring. This ramp would then be broadcastto all 201 writing columns. The verification of the writing ramp at eachcolumn requires the use of stroboscopic beam blanking, since the BSEdetector bandwidth is far below the writing ramp frequency. Thisverification process involves unblanking the beam repetitively at thesame pixel number in the ramp and then executing a “find alignment mark”function. By unblanking the beamlet at the same pointduring the ramp,and executing a “find alignment mark” function at each point, it ispossible to accurately measure the length, linearity, and orientation ofthe writing ramp at each column. The process is time consuming and couldprobably be performed only once each day without impacting throughput.On the other hand, it is possible to minimize the number of internalactive components required in the subfield driver so that reliability ishigh.

[0133] The current sensors 2116 monitor the electron beam current atvarious column electrodes. Their operation can easily be verified duringwafer exchange.

[0134] The BSE detectors 236 comprise 4 silicon PIN diodes, 4 analogpre-amplifiers, and 4 fast A/D converters. If topographical informationis desired, there are 4 BSE detector output signals for each column. Iftopographical information is not desired, then there is only one BSEdetector output signal for each column, representing the combinedsignals from the four BSE detectors 236. The BSE detectors 236 arecritical for obtaining accurate wafer alignment on each written wafer.The operation of these detectors 236 will be verified once each hour ona calibration substrate containing alignment marks, which can be imagedby all 201 BSE detectors to both verify their operation and to check(and, if necessary, reset) the column X-Y centerline.

What is claimed is:
 1. An electron optics assembly for a multi-columnelectron optical system comprising: a multiplicity of separate electronsources, such that there is a corresponding electron source for eachcolumn; a single accelerator structure situated below said electronsources; a multiplicity of separate scanning deflectors situated belowsaid accelerator structure, such that there is a corresponding scanningdeflector for each column; and a multiplicity of focus lenses situatedbelow said deflectors, such that there is a corresponding focus lens foreach column.
 2. An electron optics assembly as in claim 1, wherein eachof said electron sources comprises a multiplicity of independentlyoperable field emission cathodes.
 3. An electron optics assembly as inclaim 1, wherein said accelerator structure is comprised of a set ofaccelerator plates, a multiplicity of accelerator apertures extendingfully through said set of accelerator plates, such that there is acorresponding accelerator aperture for each column.
 4. An electronoptics assembly as in claim 1, wherein said accelerator structure iscomprised of a single piece of resistive ceramic material, amultiplicity of accelerator apertures extending fully through saidsingle piece of resistive ceramic material, such that there is acorresponding accelerator aperture for each column.
 5. An electronoptics assembly as in claim 1, further comprising a multiplicity ofalignment deflectors, for precisely steering the electron beams down thecenters of corresponding columns, situated between said electron sourcesand said accelerator structure, such that there is a correspondingalignment deflector for each column.
 6. An electron optics assembly asin claim 1, wherein said multiplicity of focus lenses are formed in asingle lens plate.